High Temperature Stable WSi 2 -Contacts on p-6H-Silicon Carbide
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ABSTRACT Amorphous tungsten-silicon layers were deposited by DC co-sputtering and subsequently annealed in an argon atmosphere up to 1325 K to form tetragonal crystalline WSi 2. Al-implanted p-6H-SiC exhibits a small depletion area forming an ohmic contact with low specific contact resistance. A modified Circular Transmission Line Model (CTLM), introduced by Marlow & Das [1] and Reeves [2], was used to characterize the electrical properties of the prepared contacts in the range between 300 K and 650 K. Deviations between calculated fieldemission contact resistances and measured contact resistances (pc= 2 -10.2 Qcm 2 , T=650 K) could be explained by TEM-cross section investigations. These deviations are caused by inhomogeneous contact interfaces originating from technological difficulties during contact preparation.
INTRODUCTION Silicon Carbide as a wide band gap semiconductor is suitable for high temperature devices with different applications. For this purpose it is necessary to obtain low-resistivity ohmic contacts on active semiconductor areas which do not degrade at high temperatures. Therefore, the contact material must have a high melting point and should not allow chemical reactions with SiC which may influence the electrical properties of the contact area. Due to theoretical calculations of a p-depletion contact on 6H SiC [3], WSi 2 is favourable as metallization material because of the high work function of 4.62 to 4.70 eV [4, 5]. The high acceptor activation energy (> 0.24 eV) causes an ionization of only approximately 1% of available acceptors at room temperature [6]. Dopant incorporation via diffusion is not feasible in SiC. However, ion implantation is a possibility to increase the acceptor concentration at the surface. This causes a narrower depletion zone which allows for thermionic-field or field emission. Fig. 1 demonstrates the influence of the hole concentration on the specific contact resistance [7, 8], using Eq. (1). k
Pc =
[42!
k- e xp qAýT
me~s 5
qh
qh
K
B
[-
/
qh
KN•A-)1](1
it a n h
FNA
[4it
rnEs
kT
( 1)
)J
Thus an ohmic contact with a low specific contact resistance should be feasible. Further research work [9] focused on the preparation and patterning of tungsten silicide metallization and on measurement of contact resistivity, applying the Circular Transmission Line Model [1, 2].
111 Mat. Res. Soc. Symp. Proc. Vol. 572 © 1999 Materials Research Society
EXPERIMENT p [cm -3]
In order to increase the acceptor concentration near the
017
surface, an Al-implantation was
performed using different doses 2 between 51014 and 61015 cm at
10.2
room temperature and an implantation energy of 50 keV. A theoretical calculation of the implantation profiles (TRIM) is shown in Fig. 2. The recrystallization of SiC was achieved by annealing at 1925 K in argon atmosphere for 20 min. To obtain high temperature stable ohmic contacts consisting of WSi 2, tungsten and silicon were DC co-sputtered in a stoichiometric ratio Si:W of 2.1:1, resulting in an amorphous layer of 400 nm thicknes
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